1. Field of the Invention
The field of the present invention relates in general to optical communication using inexpensive, stable and accurate devices for creating channels for wavelengths of light. More particularly, the field of the invention relates to a wavelength reconfigurable, multiple channel transmitter for optical Dense Wavelength Division Multiplexed signals.
2. Background
Optical fiber has nearly unlimited bandwidth, yielding 20 to 50 times more bandwidth than copper cable. Optical fiber is now the solution of choice at the wide area network (WAN) level. Implementation of fiber optic communication at the local area network (LAN) level will enable users to break current bottlenecks in the last mile of information transfer. A further attraction of fiber-optic technology is its scalability. Most current fiber LAN products are Ethernet-based in a range of 100-Mbit/second up to 1-Gbit/s. Fiber optical communication is easily scalable up to 10 Gbits/s, and several equipment vendors have announced fiber-optic links that can support transmission up to 1 terabit/s utilizing more than 128 DWDM.
Optical telecommunications networks require significantly increased bandwidth to handle current and projected communications traffic. Most optical networks use time division multiplexing (TDM) with a single laser transmitter as a means of combining many separate transmissions, allowing data rates of up to 10 Gbits/sec. The current market trend is toward systems that use many individual transmitters, each of a different wavelength to increase channel capacity (an approach known as wavelength division multiplexing, WDM). For example, a transmitter consisting of 8 distinct wavelengths provides 8xc3x97 the capacity of a single channel network (e.g. 80 Gbits/sec). Furthermore, WDM systems are scalable to 16, 32, 64, 128 etc. distinct channels, and make the most efficient use of the extremely high bandwidth of optical fiber communication networks. Therefore, what is needed is a system for providing a stable, rapidly reconfigurable, multi-channel source for WDM and DWDM (dense-WDM) optical communications networks which can meet anticipated bandwidth demand.
A fiber optic communication link provides a virtually noise free medium for transporting complex signals without distortion and interference. Fiber optic cable has losses as low as 1-2 dB per kilometer, which is much lower than the 1-2 dB per 100 ft. for coaxial cable. Due to high frequency of laser light, fiber optic cables provide a bandwidth in which many channels of information can be transported across-a single fiber cable. The laser diodes"" high efficiency, small size, high reliability, and low cost ($5-$10 for infrared, $40-$50 for visible light) make them the ideal choice for communication devices. However, other components in these devices escalate the cost of present optical DWDM transmitter devices.
Low cost optical DWDM optical communication devices are not available in the market due to the current trend toward integrating costly solid state semiconductor lasers and associated optical structures, and because optical solid state control structures are new and changing rapidly. The latest integrated optical components are created lithographically on a semiconductor chip substrate or on glass. However, it is very difficult and expensive to make multiple lasers lithographically (in VLSI) which are locked to a reference grid.
Therefore, what is needed is a way to reduce the cost of building state of the art DWDM optical communication devices. What is also needed is a method for shifting the burden of providing a high performance DWDM optical communication system from costly precision optical components to inexpensive solid state control structures. It would be desirable to achieve a high performance DWDM transmitter using low cost optical components having a wider range of tolerances than is presently possible
Conventional WDM/DWDM sources lack stabilized frequency referencing and rapid reconfigurability in the event one or more lasers fail or are disrupted. Fabry-Perot interferometers (FPIs) have been used to attempt to lock a laser to a stable frequency. Fabry-Perot Interferometers used as scanning interferometers can sense extremely small wavelength shifts when piezo-electric actuators (PZTs) are used for tuning the multi-pass dual mirror optical cavity. However, these interferometers require adequate wavelength references for long-term stability.
Molecular absorption line sources from various gases have been tried, in particular acetylene, and offer a potential medium for multi wavelength referencing. However, their unevenly spaced absorption lines and absorptive operation makes them difficult to tune for FPI-based spectral applications. FPIs with narrowlyxe2x80x94spaced uniform transmission peaks (xcx9c100 GHz) have often been considered for an absolute frequency reference when locked to atomic or molecular absorption lines. Glance, B. S. et al. (1988), xe2x80x9cDensely Spaced FDM coherent Star Network with Optical Signals Confined to Equally Spaced Frequencies,xe2x80x9d IEEE J. Lightwave Technol. LTxe2x88x926:1770-1781. and others. While there have been some applications for stabilizing laser arrays, such methods are restricted in wavelength positioning and require active feedback control in conjunction with costly precision optical structures, thereby driving up costs.
Fiber Bragg gratings (FBGs) can produce a narrow band response around a single wavelength. However, their narrow response band and over wavelength span poses limitations. Some of the current alternatives for optical transmitters provide only partial solutions. Examples of some conventional solutions and their disadvantages are described below. Most describe an optical reference source. None use a gas spectral line reference source.
U.S. Pat. No. 5,892,582 discloses a fiber Bragg grating (FBG) source which provides spectral output at a selected wavelength within a wavelength range. Thus, the FBG is used to provide a reference wherein the spectral output of the FBG marks a peak of a comb identifying its wavelength. The FBG comb is used as the reference frequency source. This approach is taken because atomic or molecular spectral lines are deemed unsuitable for the purpose of providing a stable reference source due to unevenly spaced absorption lines, and therefore are too restrictive in wavelength positioning. While this is useful in identifying and measuring wave-lengths of radiation from optical sources, it does nothing to provide an inexpensive multi channel reconfigurable optical transmitter.
U.S. Pat. No. 5,646,762 discloses another conventional approach to establishing a reference source comb using a voltage source connected to a detector and tunable etalon comb of frequencies. A digital processor connected to a photodetector and to the voltage source controls the tunable etalon. The digital processor further contains memory for storing tuning voltages for wavelengths and for storing tuning voltages for temperatures. However, this provides a costly partial approach limited to providing only a comb of uniformly spaced optical channels from an already presumed stable input reference frequency source. Furthermore, there is no provision for establishing a stable reference frequency source for the etalon comb, or to provide stability for multiple channels or reconfiguration of channels.
U.S. Pat. No. 5,949,580 discloses a controllable light amplitude divider for dividing light at a particular wavelength into two portions, which together represent the amplitude of the input light. The arrangement can be cascaded in a manner which operates as a multiplexer or demultiplexer. The main thrust of the ""580 patent is to provide a fast multiplexer and demultiplexer using an etalon. This arrangement requires costly precision optical components and is not suitable for a high performance reconfigurable DWDM application. Furthermore, the ""580 patent is not a solution for an optical transmitter, which has many other functions, and the multiplexer/demultiplexer scheme is not necessary for a most optical transmitters
U.S. Pat. No. 4,813,756 discloses a device for interconnecting or for linking two optical fibers comprising a mechanically rotatable etalon arrangement. The ""756 patent principally creates a device for interconnecting two optical fibers. While multiple etalons are used in various configurations, this does not provide a full multichannel optical transmitter, but rather a partial and very expensive way to connect DWDM fiber. ""756 also claims optical channel selection filter mounted for coupling two single mode optical fibers.
U.S. Pat. No. 4,707,061 discloses an optical communications system using a resonant cavity for supporting a set of resonant modes and introducing predetermined reference wavelengths. A means for controlling the resonant cavity tunes one resonant mode to the wavelength of a fixed wavelength light source. Semiconductor laser sources are used in conjunction with resonant cavities which must be present at the transmitter as well as the receiver, thereby adding expense and complexity to an all ready cumbersome scheme to communicate over an optic network.
U.S. Pat. No. 5,673,129 discloses a plurality of optical transmitters for outputting optical signals, at least one optical wavelength selector communicating with the optical transmission. The wavelength selector includes a Bragg grating member with a wavelength band of high reflectivity. The wavelength band of high reflectivity for each Bragg grating member corresponds to an optical channel output. The ""129 patent provides a closed loop optical system and uses semiconductor lasers, thereby resulting in a costly approach and complex system using gain bands, amplifier stages and pumps.
U.S. Pat. No. 6,014,237 discloses a multi wavelength mode-locked (MWML) laser source including a semiconductor optical amplifier (SOA) disposed in a cavity of the MWML laser source. The SOA is actively driven by a radio frequency (RF) signal and emits periodic pulses within a plurality of discrete wavelength bands simultaneously. The semiconductor optical amplifier and radio frequency (RF) signal driver make this a relatively expensive solution. The ""237 patent also requires that input signals be multiplexed by a high speed electronic time domain multiplexer (ETDM) to a higher bit-rate electronic data stream for coding by an optical modulator in the optical pulse stream emitted by the MWML-DWDM. This results in complex design and interfacing requirements which are not suited to a practical, low cost implementation.
U.S. Pat. No. 6,044,189 discloses a temperature compensating fiber Bragg grating contained in an optical fiber. The ""189 patent is concerned with the improvement of control of a fiber Bragg grating, only one possible component of a DWDM system.
U.S. Pat. No. 6,028,881 discloses a pump source tunable among a plurality of pumping wavelengths; a plurality of waveguide lasers responsive to respective pumping wavelengths for emitting light. The ""881 patent introduces a method of combining solid state waveguide lasers with semiconductor lasers and enhancing electrical tunability. Components include intra-cavity pumping and pump reflectors. These require expensive integrated optics to manufacture.
U.S. Pat. No. 5,953,139 discloses an analog light wave communication system having at least two optical transmitters. The first WDM receives optical information signals from the optical transmitters and multiplexes the optical information signals to a composite optical signal at an output. Each input of the WDM comprises at least one optical resonant cavity; an oscillator circuit providing a single tone modulation signal and a phase modulator having an optical input coupled to the output of the WDM. The single tone modulation signal drives a composite optical signal which is too restrictive for most optical transmitter uses.
Conventional WDM/DWDM sources fail to provide the stabilized spectral frequency referencing or the rapid optical channel reconfigurability necessary for a high performance, low cost optical DWDM system. The design of conventional optical transmitters is directed toward the use of integrated optics wherein all optical components are created lithographically on a semiconductor chip substrate or on glass. These can be difficult and expensive to manufacture, as multiple lasers must be locked to a reference grid lithographically, using VLSI techniques.
Conventional DWDM approaches are not cost-competitive in the metro markets where they must compete with lower cost all electronic products which are more dynamic and operate at lower bandwidth demands. Most conventional DWDM systems operate at OC-48. As demand for higher bandwidth increases, these data rate demands will migrate to Metro markets. Therefore, what is needed are low cost optical devices which are dynamic from a standpoint of configurability and can meet higher bandwidth demands.
DWDM optical devices demand very large bandwidths. A failure of one optical channel affects multiple protocol stack layers and thus large numbers of users. Therefore, in the event of a failure at the channel level, restoration must occur at multiple stack levels. The speed of restoration is critical. Bandwidth reservation is an option for these slower layers, but this requires that excess idle capacity must be built in at the optical device level.
In the protocol layer scheme for optical channels, the Internet Protocol (IP) layer is carried by the ATM layer below. IP over DWDM presents topology node architecture issues. There is a virtual mapping between the physical and logical topology of IP over ATM, which leads to scaling challenges. One of the solutions is-to make every switch into a router. The current cost of optical routers makes this a very expensive solution. What is needed are inexpensive optical switches and routers.
In high reliability networks, reconfigurability options are necessary. However, conventional designs are very expensive. For example, each channel has a fixed-frequency laser, in addition to the laser that carries the data, the xe2x80x9cactivexe2x80x9d laser. The fixed frequency laser is a spare, identical to the active laser that initially carries the modulated signal. The spare laser must be prepared to carry the modulated signal on the active laser""s specific carrier frequency. Both lasers are generally physically located in the same rack.
Thus, in the event of a laser failure, the spare fixed-frequency laser takes functional control on only the designated frequency and signal. However, this approach forces the manufacturer to fabricate a redundant number of lasers, at least one spare for each active laser, to ensure a high reliability device. Therefore, what is needed is an optical channel device which requires fewer lasers, lower production cost and provides high reliability. What is also needed is a DWDM transmitter which is reconfigurable, not only with respect to lasers, but also across channels.
In order to overcome the foregoing deficiencies in conventional optical DWDM systems, an aspect of the invention provides a high performance reconfigurable DWDM transmitter incorporating low cost discrete optical components which can be placed in v grooves etched in a silicon optical micro-board or the like, keeping costs of manufacturing low. The lasers are packaged in modules based on the technology of meso scale optics. The physical size of a multi channel module is no bigger than a conventional single laser module.
In another aspect of the invention, direct wavelength monitoring is achieved by using a wavelength modulation locking technique applied independently to a gas absorption line and to etalon fringes. The frequency stability and resolution achieved thereby make it possible to pack channels closely and achieve spacing up to the modulation limit, filling the available bandwidth. This now enables high density DWDM to populate as many channels as desired to the modulation limit.
An aspect of the invention uses a set of n lasers and k spare sources, wherein each laser is actively locked to a set of equally spaced wavelengths according to the ITU frequency grid, and simultaneously to a stable spectral reference wavelength. The set of equally spaced frequencies is generated by an etalon, acting as a frequency comb generator. The absolute wavelength standard is provided by a gas absorption cell. The wavelength of each channel can be changed on a millisecond (msec) time scale under microprocessor control in the event that any channel should fail, thereby enabling substantially instantaneous reconfigurability.
According to this aspect of the invention, a separate (fixed frequency) spare laser is not needed for each active (fixed frequency) laser. The invention enables a single laser to be used as a substitute for a number of fixed-frequency lasers, and a number of fixed-frequency spare lasers as well.
In addition, any xe2x80x9cactivexe2x80x9d laser (i.e. a laser already assigned to a particular channel, not a spare) can be reassigned to a different channel, if necessary, which further improves network reliability. If all spare lasers within a module should fail, and an active laser at the most valuable channel fails as well, the system still can carry the traffic over the most valuable channel by reassigning a laser from one of a lesser used or less valuable channels to the most valuable channel, until physical replacement of lasers is made. The modulation signal is switched electronically to modulate the spare laser instead of the laser that failed